1. Field of the Invention
This invention relates to a diffraction optical element, an optical system and an optical device.
2. Related Background Art
Conventionally, the optical systems of semiconductor manufacturing apparatus including projection optical systems are mostly formed by using only refraction optical elements. However, in recent years, a number of optical systems utilizing diffraction optical elements (DOEs) have been proposed. Japanese Patent Application Laid-Open No. 10-239508 describes a diffraction optical element to be used for a lens in an optical system of the type under consideration that provides an improved diffraction efficiency and can be manufactured with ease.
However, the proposals that have been made to date for diffraction optical elements to be used for a lens are mainly intended to improve the diffraction efficiency and the ease of manufacture and do not take the environmental durability into consideration. For instance, a diffraction optical element disclosed in the above Japanese Patent Application Laid-Open No. 10-239508 does not show a level of environmental durability that is high enough to make it pass an environmental durability test such as a weather resistance test where it is exposed to an intense irradiation of ultraviolet rays.
In view of the above identified technological problem, it is therefore the object of the present invention to provide a diffraction optical element, an optical system and an optical device that show a high environmental durability.
According to the invention, the above object is achieved by providing an diffraction optical element comprising:
a substrate;
a diffraction grating formed on said substrate and made of a material with low ultraviolet resistance; and
an ultraviolet screening means arranged on said substrate at a position closer to the incident light receiving side of the optical element relative to said diffraction grating.
In another aspect of the invention, there is provided an optical system comprising:
a first optical element including a substrate and a diffraction grating formed on said substrate and made of a material with low ultraviolet resistance; and
a second optical element arranged at a position closer to the incident light receiving side of the system relative to said first optical element and having an ultraviolet screening effect.
In still another aspect of the invention, there is provided an optical device adapted to form an image by means of an optical system according to the invention.